Method for reducing rag layer volume in stationary froth treatment

ABSTRACT

A method for reducing rag layer volume in a stationary bitumen froth treatment process is provided, comprising subjecting dilfroth having a naphtha diluent to bitumen ratio of about 0.7 to gravity settling in a splitter vessel to produce an overflow stream of raw dilbit and an underflow stream of splitter tails; mixing the splitter tails with a naphtha diluent to give a mixture having a naphtha diluent to bitumen ratio of less than about 6:1 in a scrubber feed tank; and subjecting the mixture to gravity settling and agitation in a scrubber vessel to produce an overhead stream of scrubber hydrocarbons and an underflow stream of scrubber tails.

FIELD OF THE INVENTION

The present invention relates to a method for reducing rag layer volumein a stationary bitumen froth treatment process by agitating naphthadiluted bitumen froth and using a low naphtha to bitumen ratio atspecific treatment stages in a stationary froth treatment process.

BACKGROUND OF THE INVENTION

Oil sand, as known in the Athabasca region of Alberta, Canada, compriseswater-wet, coarse sand grains having flecks of a viscous hydrocarbon,known as bitumen, trapped between the sand grains. The water sheathssurrounding the sand grains contain very fine clay particles. Thus, asample of oil sand, for example, might comprise 70% by weight sand, 14%fines, 5% water and 11% bitumen (all % values stated in thisspecification are to be understood to be % by weight).

For the past 25 years, the bitumen in Athabasca oil sand has beencommercially recovered using a water-based process. In the first step,the oil sand is slurried with process water, naturally entrained airand, optionally, caustic (NaOH). The slurry is mixed, for example in atumbler or pipeline, for a prescribed retention time, to initiate apreliminary separation or dispersal of the bitumen and solids and toinduce air bubbles to contact and aerate the bitumen. This step isreferred to as “conditioning”.

The conditioned slurry is then further diluted with flood water andintroduced into a large, open-topped, conical-bottomed, cylindricalvessel (termed a primary separation vessel or “PSV”). The diluted slurryis retained in the PSV under quiescent conditions for a prescribedretention period. During this period, aerated bitumen rises and forms afroth layer, which overflows the top lip of the vessel and is conveyedaway in a launder. Sand grains sink and are concentrated in the conicalbottom. They leave the bottom of the vessel as a wet tailings streamcontaining a small amount of bitumen. Middlings, a watery mixturecontaining fine solids and bitumen, extend between the froth and sandlayers.

The wet tailings and middlings are separately withdrawn, combined andsent to a secondary flotation process. This secondary flotation processis commonly carried out in a deep cone vessel wherein air is spargedinto the vessel to assist with flotation. This vessel is referred to asthe TOR vessel. The bitumen recovered by flotation in the TOR vessel isrecycled to the PSV. The middlings from the deep cone vessel are furtherprocessed in induced air flotation cells to recover contained bitumen.

The bitumen froths produced by the PSV are subjected to cleaning, toreduce water and solids contents so that the bitumen can be furtherupgraded. More particularly, it has been conventional to dilute thisbitumen froth with a light hydrocarbon diluent, for example, withnaphtha, to increase the difference in specific gravity between thebitumen and water and to reduce the bitumen viscosity, to thereby aid inthe separation of the water and solids from the bitumen. This diluentdiluted bitumen froth is commonly referred to as “dilfroth”. It isdesirable to “clean” dilfroth, as both the water and solids pose foulingand corrosion problems in upgrading refineries. By way of example, thecomposition of naphtha-diluted bitumen froth typically might have anaphtha/bitumen ratio of 0.65 and contain 20% water and 7% solids. It isdesirable to reduce the water and solids content to below about 3% andabout 1%, respectively.

Separation of the bitumen from water and solids may be done by treatingthe dilfroth in a sequence of scroll and disc centrifuges.Alternatively, the dilfroth may be subjected to gravity separation in aseries of inclined plate separators (“IPS”) in conjunction withcountercurrent solvent extraction using added light hydrocarbon diluent.However, these treatment processes still result in bitumen oftencontaining undesirable amounts of solids and water.

More recently, a staged settling process (often referred to asStationary Froth Treatment or SFT) for cleaning dilfroth was developedas described in U.S. Pat. No. 6,746,599, whereby dilfroth is firstsubjected to gravity settling in a splitter vessel to produce a splitteroverflow (raw diluent diluted bitumen or “dilbit”) and a splitterunderflow (splitter tails) and then the raw dilbit is further cleaned bygravity settling in a polisher vessel for sufficient time to produce anoverflow stream of polished dilbit and an underflow stream of polishersludge. Residual bitumen present in the splitter tails can be removed bymixing the splitter tails with additional naphtha and subjecting theproduced mixture to gravity settling in a scrubber vessel to produce anoverhead stream of scrubber hydrocarbons, which stream is recycled backto the splitter vessel.

However, a rag layer tends to form between the bitumen phase and thetailings phase in the scrubber vessel during gravity settling of thesplitter tails/naphtha mixture, and to a lesser extent, in the polishervessel during gravity settling of the raw dilbit. It is believed thatthe rag layer may be a result of stable water-in-oil emulsionspersisting, primarily due to the clay solids present in the dilutedbitumen froth. The rag layer is a mixture of partially oil-wet solids,oil and water-in-oil emulsions. Much of the clay solids are kaoliniteand illite. The formation of such a rag layer prevents completeseparation of the diluted bitumen from the water and solids, reducesdewatering, and depresses bitumen recovery.

Accordingly, there is a need for a method of reducing and/or breakingthe rag layer in stationary bitumen froth treatment processes.

SUMMARY OF THE INVENTION

The current application is directed to a method of reducing rag layervolume in stationary bitumen froth treatment processes. It wassurprisingly discovered that by conducting the method of the presentinvention, one or more of the following benefits may be realized:

(1) Mixing of the rag layer that forms in a separation vesselsignificantly reduces the rag layer volume. In particular, rag layermixing alone significantly reduces rag layer volume compared to feed(e.g., scrubber feed) mixing alone. Gentle or mild mixing is sufficient.The combined use of rag layer mixing and scrubber feed mixing is moreeffective in reducing the rag layer volume compared to either rag layermixing alone or feed mixing alone.

(2) Use of a low scrubber naphtha to bitumen ratio (less than about 4:1,preferably less than about 3:1) for the scrubber feed contributes to afurther reduction in rag layer volume by minimizing the precipitation ofasphaltenes which normally stabilize the rag layer.

(3) Reduction in rag layer volume is optimally achieved by combining ragmixing, scrubber feed mixing, and a low naphtha to bitumen ratio for thescrubber feed, without necessitating silicate addition to the bitumenfroth or rag water addition to the scrubber.

(4) Combining rag mixing, scrubber feed mixing, and a low naphtha tobitumen ratio for the scrubber feed yielded a scrubber product having abitumen content greater than about 20 wt % and a solids content lessthan about 5 wt %. The enhancement in scrubber product quality reducesthe amount of water and solids recycled to the splitter feed, thereby,in turn, improving splitter product quality.

(5) Agitation of the bitumen froth at various treatment stages withingravity settlers including for example, the scrubber feed tank, scrubberand polisher, may reduce the rag layer volume.

Thus, use of the present invention may improve bitumen recovery andproduct quality by effectively reducing the rag layer volume.

In one aspect, a method of reducing rag layer volume in a stationarybitumen froth treatment process is provided, comprising:

-   -   subjecting dilfroth having a naphtha diluent to bitumen ratio of        about 0.7 to gravity settling in a splitter vessel to produce an        overflow stream of raw dilbit and an underflow stream of        splitter tails;    -   mixing the splitter tails with a naphtha diluent to give a        mixture having a naphtha diluent/bitumen ratio of less than        about 6:1; and    -   subjecting the mixture to gravity settling and agitation in a        scrubber vessel to produce an overhead stream of scrubber        hydrocarbons and an underflow stream of scrubber tails.

In one embodiment, the method further comprises subjecting the rawdilbit to gravity settling and agitation in a polisher vessel to producean overflow stream of polished dilbit and an underflow stream ofpolisher sludge.

In one embodiment, the naphtha diluent to bitumen ratio of the mixtureis less than 4:1. In another embodiment, the naphtha diluent to bitumenratio of the mixture is less than or equal to 3:1.

In one embodiment, mixing reduces the rag volume in a polisher vessel atnaphtha diluent to bitumen ratio of about 0.7.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings wherein like reference numerals indicatesimilar parts throughout the several views, several aspects of thepresent invention are illustrated by way of example, and not by way oflimitation, in detail in the figures, wherein:

FIG. 1 is a graph showing, in general, one embodiment of a bitumen frothtreatment process useful in the present invention.

FIG. 2 is a graph showing the rag layer volume (mL) for each testcondition.

FIG. 3 is a graph showing the rag layer solids content (mass %) for eachtest condition.

FIG. 4 is a graph showing the rag layer water content (mass %) for eachtest condition.

FIG. 5 is a graph showing the rag layer bitumen content (mass %) foreach test condition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various embodiments of thepresent invention and is not intended to represent the only embodimentscontemplated by the inventor. The detailed description includes specificdetails for the purpose of providing a comprehensive understanding ofthe present invention. However, it will be apparent to those skilled inthe art that the present invention may be practiced without thesespecific details.

The present invention relates generally to a method of reducing and/orbreaking rag layer in a stationary bitumen froth treatment process. Themethod includes agitating bitumen froth and using a low naphtha tobitumen ratio at specific stages of the froth treatment process.

FIG. 1 is a general schematic of a stationary bitumen froth treatmentprocess using gravity settlers, which can be used in one embodiment ofthe present invention. Bitumen froth 10 is initially received from anextraction facility which extracts bitumen from oil sand using a waterextraction process known in the art. The bitumen froth 10, as received,typically comprises about 60% bitumen, about 30% water and about 10%solids.

A hydrocarbon diluent 12 is mixed with bitumen froth 10 in a suitablemixer 14 to provide diluent-diluted bitumen froth (referred to herein as“dilfroth”) 16. In one embodiment, the hydrocarbon diluent 12 isnaphtha. The naphtha is supplied in an amount such that the naphtha tobitumen ratio of the dilfroth 16 is preferably in the range of 0.5 to1.0, most preferably about 0.7.

As used herein, the term “silicate” refers to any of a wide variety ofcompounds containing silicon, oxygen and one or more metals with orwithout hydrogen, for example, a sodium silicate having the generalformula xNa₂O.ySiO₂. Silicates are known to change the surfaceproperties of fine solids, causing them to associate with the waterphase, rather than the oil phase. A silicate 18 is typically added tothe dilfroth 16 at a concentration ranging between about 0.0001 to about0.1% wt/wt or more. However, in the present invention, reduction of therag layer volume may be achieved without the addition of silicate 18. Inone embodiment, addition of silicate 18 to the bitumen froth 10 isoptional. The dilfroth 16 may be fed into an agitated feed tank 20, forexample, a splitter feed tank.

The agitated dilfroth 22 is then pumped into the chamber of a gravitysettler vessel or splitter 24 having a conical bottom 26, and underflowand overflow outlets 28, 30 at its bottom and top ends, respectively.The dilfroth 22 is temporarily retained in the splitter 24 for asufficient length of time to allow a substantial portion of the solidsand water to separate from the diluted bitumen. The splitter overflow isreferred to as raw dilbit 32. Line 34 withdraws a stream of splittertails 36 through the underflow outlet 28. Splitter overflow line 38collects an overflow stream of raw dilbit 32.

The bottom layer of splitter tails 36 comprises mainly sand and aqueousmiddlings, and some hydrocarbons, and the top layer of raw dilbit 32comprises mainly hydrocarbons containing some water and a reduced amountof fines (clay particles).

The raw dilbit 32 produced through the splitter overflow outlet 30routinely comprises less than about 3% solids, and may be pumped to asecond gravity settler vessel or polisher (40) following optionaladdition of a demulsifier to enhance water separation, and subjected tofurther gravity settling therein. The polisher is operated at naphtha tobitumen ratio of about 0.7. Water droplets coalesce and settle, togetherwith most of the remaining fine solids. Since a rag layer may formduring gravity settling, the raw dilbit 32 is thus agitated while beingretained within the polisher 40 to reduce the rag layer volume. Polisherdilbit 42, comprising hydrocarbons, typically containing <3.0 wt. %water and <1.0 wt. % solids, is removed as an overflow stream from thepolisher 40. Polisher sludge 44, comprising water, solids and typicallybetween about 20-70% hydrocarbons, or 12-40% bitumen, is removed fromthe polisher 40 as an underflow stream.

The splitter tails 36 produced through the splitter underflow outlet 28are pumped through line 46, to an agitated feed tank 48 or scrubber feedtank, where it may be mixed with polisher sludge 44 and naphtha 12 toproduce a scrubber feed 50 preferably having a naphtha to bitumen ratioless than about 4:1. In one embodiment, the naphtha to bitumen ratio isless than about 3:1. The use of a naphtha to bitumen ratio less thanabout 4:1 prevents the precipitation of asphaltenes which normallystabilize the rag layer. The rag emulsion is rendered weaker and easierto break down through agitation of the scrubber feed 50 with agitator66. In one embodiment, agitation is conducted at a speed in the range ofabout 700 rpm to about 1300 rpm, preferably about 700 rpm.

The agitated scrubber feed 50 is then introduced to a third gravitysettler vessel or scrubber 52. The scrubber feed 50 is then temporarilyretained in the scrubber 52 (for example for 20 to 30 minutes) andsubjected to gravity settling therein. A stable rag layer typicallyforms between the diluted bitumen layer and the water layer in thescrubber 52 during gravity settling of the scrubber feed 50. Thescrubber feed 50 is agitated with agitator 64 while being retainedwithin the scrubber 52. Without being bound to theory, it is believedthat agitation induces shear, which minimizes rag layer volume andbreaks the gel-like rag layer, but not the water-in-oil emulsion whichis present in the oil and water interface. In one embodiment, agitationis conducted at a speed in the range of about 52 rpm to about 188 rpm,preferably about 52 rpm.

Without being bound by theory, it is believed that addition of water 54to the rag layer removes fine solids; however, in the present invention,reduction of the rag layer volume may be achieved without the additionof water 54 to the rag layer within the scrubber 52. In one embodiment,addition of water 54 to the rag layer is optional.

The scrubber overflow stream 56 of hydrocarbons, mainly comprisingnaphtha and bitumen, is removed through an overflow outlet 58 and in oneembodiment may be recycled through line 60 to the mixer 14. Scrubberunderflow stream of scrubber tails 62, comprising water and solidscontaining some hydrocarbons, is removed and forwarded to a naphtharecovery unit (not shown).

Exemplary embodiments of the present invention are described in thefollowing Example, which is set forth to aid in the understanding of theinvention, and should not be construed to limit in any way the scope ofthe invention as defined in the claims which follow thereafter.

Example 1

The flow sheet used for the evaluation of the rag volume reduction isessentially the same as that shown in FIG. 1 except that the polishervessel was omitted to enable timely experimentation. Five variablesincluding silicates concentration, water addition to rag layer, raglayer agitation, scrubber feed agitation and scrubber N/B ratio wereevaluated using a 2⁵⁻¹ fractional factorial design resulting in 16different experimental run conditions. Table 1 summarizes the range ofthe independent variables and the test matrix. In addition, Table 1includes repeat conditions and an additional run using higher rag layeragitation (Condition No. 18), resulting in a total of 20 runs completed.

TABLE 1 Test Matrix Water Rag Layer Silicates Addition AgitationScrubber Feed Scrubber Condition wt. % (g/min) RPM RPM N/B 1 0 0 0700 >6 2 0 16 52 700 >6 3 0.1 0 52 700 >6 4 0.1 16 0 700 >6 5 0.1 16 521300 >6 6 0 0 52 1300 >6 7 0 16 0 1300 >6 8 0 0 52 700 <3 9 0 16 0 700<3 10 0 0 0 1300 <3 11 0 16 52 1300 <3 12 0.1 0 52 1300 <3 13 0.1 16 01300 <3 14 0.1 0 0 1300 >6 15 0.1 0 0 700 <3 16 0.1 16 52 700 <3 17 0 00 700 >6 18 0 0 188 700 >6 19 0 0 0 700 >6 20 0.1 16 52 700 <3

In this evaluation, controlling the rag layer growth in the scrubbervessel is the primary objective. Quantifying whether the rag layer hasbeen reduced or changed is done by measuring the rag volume and raglayer composition. It is desirable for the rag to occupy less volume inthe scrubber, which directly implies that there is physically less raglayer present in the scrubber. The rag layer composition is not aconcern under steady state conditions, provided the rag layer is notgrowing. Therefore, the following analysis focuses on the rag layergrowth, i.e., the rag layer's volume and not its composition. Thus, theexperimental design evaluates the effect of the five variables on raglayer volume.

The amount of rag layer produced varied considerably with the variousconditions and, in some instances, there was over an order of magnitudedifference in rag layer volume, from 71 to 780 mL. The volume of raglayer produced in each condition is summarized in FIG. 2. Fiveconditions (9, 10, 14, 17 and 19) produced the largest rag volume,wherein all five conditions did not have rag mixing. Conditions 2, 4, 8,16, and 20 produced some of the lowest amounts of rag volume; these fiveconditions involved either rag mixing or rag water addition or both. Thevariability of the measured rag volume based on repeats was 31% relativeerrors. Based on 95% confidence limits, the maximum and minimum ragvolumes are significantly different.

Rag layer composition appears to be more variable among the variousconditions tested, as shown in FIGS. 3, 4, and 5. The results appear toindicate that the rag layer bitumen content increased when the scrubberN/B was lowered (FIG. 5). Otherwise, no particular trend of thecomposition with operating conditions is observed.

The effects of the five rag layer mitigation variables on the rag layervolume reduction were evaluated using an experimental design softwarepackage (Design Expert® by Stat-Ease). This software enabled the use ofall experimental data, including repeats, to produce parameter estimatesand determine the significant of the parameter estimates at 95%confidence limits. The empirical model representing rag volume, Y₁ is:

Y ₁=310−57X ₂−110X ₃+65X ₁ X ₃+47X ₁ X ₄+72X ₂ X ₃−82X ₂ X ₅ R ²=0.91

Note that X_(i) represents coded value of independent variable i. Theresults show that there are two main effects and four two-factorinteraction effects to be significant at 95% confidence limits. Themodel has a R² of 91%, which means that 91% of the data variation can beexplained by the model. Among these effects, rag layer mixing (X₃) hasthe most significant effect on the rag volume, i.e. high level of mixingreduced the rag volume. The other main effect is rag water addition,where addition of rag water reduced rag layer volume. The interpretationof two factor interaction effects is as follows:

-   X₁X₃: This term represents the interacting effect between the    silicate and rag mixing. The interacting effect needs to be    minimized to achieve a reduction in rag volume. Therefore the sign    of these two variables must be opposite. Since the rag mixing has    been assigned a positive value above, the silicate addition will be    negative. As a result, the reduction of rag volume can be achieved    through rag mixing and with no silicate addition.-   X₁X₄: This term represents the interacting effect between the    silicate and the Scrubber feed mixing. Similarly, the sign of the    two variables needs to be opposite for rag volume reduction. Since    the sign of the silicate is negative, the Scrubber feed mixing will    be positive. Therefore, the reduction of rag volume required no    silicate addition and high Scrubber feed mixing.-   X₂X₃: This term represents the interacting effect between rag water    addition and rag mixing. Again, the sign of the two variables need    to be opposite to achieve the rag volume reduction. However, the    main effect of these two variables suggests the sign to be the same.    Under this scenario, magnitude of the three terms (X₂, X₃, and X₂X₃)    required to be evaluated and optimized. If the rag volume is    minimized, the results show that rag mixing must remain positive,    but the sign of rag water addition will have to change to be    negative.-   X₂X₅: This interacting effect is between rag water addition and    Scrubber N/B ratio. The sign of the two variables should be the same    to allow the reduction of the rag. If the sign of rag water addition    is negative, the sign of Scrubber N/B ratio should also be negative.    The results suggest that a N/B ratio less than or equal to 3 should    be to use to decrease the rag volume. This is not unexpected as    N/B>4 would precipitate bitumen asphaltenes, which stabilize water    in oil emulsion and hence the stability of the rag layer.

Based on the above evaluation, for rag volume reduction, the recommendedrag mitigation variable settings are: no silicate and rag wateraddition, high rag and Scrubber feed mixing and low N/B ratio. Usingthese variables settings and the developed model (equation shown above),the rag layer volume can be estimated. Table 2 focuses on the mixingeffects on rag layer volume. The standard flow sheet conditions wereused, which required the N/B ratio to be greater than or equal to 6 andthe scrubber feed mixing was set at 700 rpm to prevent water and solidssettling in the scrubber feed tank. The rag layer volume withoutadditional mixing introduced to the system was estimated to be 743 mL.Increasing the scrubber feed mixing from 700 to 1300 rpm reduced the raglayer volume to 649 mL. Addition of the rag layer mixing at 52 rpmsignificantly decreased the rag layer volume to 249 mL. The use of raglayer mixing at 52 rpm and an increase of scrubber feed mixing to 1300rpm further decreased the rag volume to 155 mL. These results clearlydemonstrated the impact of mixing on rag layer reduction.

TABLE 2 Variable Rag volume, mL Base case (no rag mixing and scrubberfeed mixing 743 at 700 rpm) Base case + Scrubber feed mixing (1300 rpm)649 Base case + rag mixing (52 rpm) 249 Base case + rag mixing (52rpm) + Scrubber feed 155 mixing (1300 rpm)

Without being bound to theory, a possible explanation as to why therecommended rag mitigation variable settings worked in the reduction ofrag layer volume is offered as follows. The rag layer is comprised ofmultiple emulsions, which are stabilized by solids and/or bitumenasphaltenes. The majority of solids are hydrophobic solids as result ofsurface property change due to the interaction between the hydrophilicclays and naphthenic acid. The clays and natural surfactants are presentnaturally in oil sands and process water. It was proposed that additionof silicate could change the solids surface properties from hydrophobicto hydrophilic. However, it was found that silicate is not required forthe rag layer volume reduction; these results may suggest that solidspresent in rag layer may not have originated from hydrophobic solids andthey may be from the organic rich solids like humic matter. Wateraddition to the rag layer was hypothesized to remove the convertedhydrophobic clay solids. Since the hypothesized hydrophilic clay solidsdo not seem present, addition of rag water is, therefore, not required.Both the scrubber feed mixing and rag layer mixing are used to break theemulsion and, hence, reduce the rag layer volume. The use of lowscrubber N/B ratio prevents the precipitation of asphaltenes, hence, therag emulsion is weaker and rag volume can be easier to break down byshear through mixing.

Example 2

An experimental condition was conducted to determine the impact ofhigher rag layer mixing on rag layer volume reduction. The only variablethat changed was to increase the rag layer mixer speed to 188 rpm. Allother rag layer mitigation variables were set at base case flow sheetconditions, i.e., no silicate or rag water addition, low scrubber feedmixer speed and high scrubber N/B ratio of greater than or equal to 6.

The rag volume comparison for the three rag mixer speeds is shown inTable 3. The rag layer volume at rag mixer speed of 52 rpm is notsignificantly different from the rag layer volume at rag layer mixerspeed of 188 rpm. However, the rag layer volume at base case conditionis significantly different from the rag layer volume at rag mixer speedsof both 52 rpm and 188 rpm.

TABLE 3 Rag Variable Volume, ml Base case 740 Base + rag layer mixing at52 rpm 249 Base + high rag layer mixing at 188 rpm 273The results in Table 3 show that rag mixing (52 to 188 rpm) didsignificantly reduce the scrubber rag layer volume compared with thebase case. A higher rag mixer speed did not appear further reduce therag layer volume. The impact of the shear on the scrubber rag layer wasable to reduce the rag layer volume only to a certain extent. Othervariables to minimize the formation of emulsion stabilizer, such asscrubber N/B ratio, are also important in the rag layer volumereduction.

The scope of the claims should not be limited by the preferredembodiments set forth in the examples, but should be given the broadestinterpretation consistent with the description as a whole, whereinreference to an element in the singular, such as by use of the article“a” or “an” is not intended to mean “one and only one” unlessspecifically so stated, but rather “one or more”. All structural andfunctional equivalents to the elements of the various embodimentsdescribed throughout the disclosure that are known or later come to beknown to those of ordinary skill in the art are intended to beencompassed by the elements of the claims. Moreover, nothing disclosedherein is intended to be dedicated to the public regardless of whethersuch disclosure is explicitly recited in the claims.

We claim:
 1. A method for reducing rag layer volume in a stationarybitumen froth treatment process comprising: subjecting dilfroth having anaphtha diluent to bitumen ratio of about 0.7 to gravity settling in asplitter vessel to produce an overflow stream of raw dilbit and anunderflow stream of splitter tails; mixing the splitter tails with anaphtha diluent to give a mixture having a naphtha diluent to bitumenratio of less than about 6:1 in a scrubber feed tank; and subjecting themixture to gravity settling and agitation in a scrubber vessel toproduce an overhead stream of scrubber hydrocarbons and an underflowstream of scrubber tails.
 2. The method of claim 1, further comprisingsubjecting the raw dilbit to gravity settling and agitation in apolisher vessel at a naphtha diluent to bitumen ratio of about 0.7 toreduce and/or break rag layer and to produce an overflow stream ofpolished dilbit and an underflow stream of polisher sludge.
 3. Themethod of claim 1, further comprising diluting bitumen froth with theproduced scrubber hydrocarbons to form the dilfroth.
 4. The method ofclaim 1, wherein the mixture has a naphtha diluent to bitumen ratio ofless than 4:1.
 5. The method of claim 1, wherein the mixture has anaphtha diluent to bitumen ratio of less than or equal to 3:1.
 6. Themethod of claim 1, wherein agitation in the scrubber feed tank isconducted in the range of about 700 rpm to about 1300 rpm.
 7. The methodof claim 6, wherein agitation in the scrubber feed tank is conducted atabout 700 rpm.
 8. The method of claim 1, wherein agitation in thescrubber vessel is conducted in the range of 1 rpm to about 188 rpm. 9.The method of claim 8, wherein agitation in the scrubber vessel isconducted at about 52 rpm to about 188 rpm.
 10. The method of claim 1,wherein agitation in the scrubber feed tank is conducted at about 1300rpm and agitation in the scrubber vessel is conducted at about 52 rpm.11. The method of claim 3, wherein optionally, silicate is added to thebitumen froth.
 12. The method of claim 1, wherein optionally, water isadded to the scrubber vessel.